The present invention relates generally to optical inspection systems and more particularly to optical instruments for measuring defects on a finished surface.
The semiconductor industry increasingly requires ever smaller semiconductor structures. This is due to the increased need for more capacity per chip. More capacity includes faster response times and more function per unit of area of each chip. Presently, the semiconductor industry projects an immediate move to manufacture 64 Mb, and in the near future 256 Mb, DRAMS. This increased structure density requires the detection and identification of smaller particles and defects that would render the wafer useless for this application. Detection of 2 nm substrate defects and 20 nm sized particles on unpatterned silicon wafers is necessary.
In addition, the industry is changing from 200 mm to 300 mm diameter wafers with fewer defects and rapid detection at all processing stages. To meet these needs, defect data must be processed in near-real-time to expedite correction of processing problems through statistical process control techniques.
Many surface roughness inspection systems are available; for example, high resolution microscopes such as the atomic force microscope and optical microscopes such as the phase contrast microscope. Other optical measurement systems such as ellipsometers, and mechanical contact methods that use a stylus are also used to measure surface roughness. For sub-micron resolution, most of these techniques are not suitable for in-process surface inspection. High resolution microscopes require cumbersome surface preparations and expensive operations. Standard optical microscopes do not have sufficient resolution and accuracy. Ellipsometry or spectroscopy also do not provide adequate surface roughness information. Mechanical stylus devices may damage the surface and thus are not even considered.
An additional possibility is the optical heterodyne (frequency-shifted) microscope. The heterodyne microscope is an interferometric microscope where a signal beam is frequency-shifted relative to a reference beam. The signal containing the optical phase information (i.e., surface roughness) can be electronically detected by comparing the phase of the beams from different portions of the water surface. Optical heterodyne microscopes can calculate the surface roughness to less than 0.1 nm. Optical heterodyne microscopes, however, have several drawbacks, including inadequate lateral resolution, slow scanning speed, critical focusing requirements of the beams, limited dynamic range, and inadequate information on the composition of surface defects. A serious drawback is in assessing the number of defects over a large wafer by scanning with a micron-sized area of view. This would require hours to examine a wafer even with a multiple array of detectors. Such slow processing time is unacceptable for in-process inspection for the semiconductor industry.
Conventional scatterometry is another method for measuring defects. The limitation with conventional scatterometry is that the intensity of scattering is measured at oblique angles, excluding the specular beam. Under this condition, the diameter of particles that can be detected must be greater than 100 nm (or at best 80 nm). Conventional scatterometry can rapidly scan large wafers to measure larger particle sizes, but has the disadvantage that the size of particles that can be measured is not small enough for the smaller structures to be built in the near future.